1. Field of the Invention
This invention relates to illumination systems for optical readers such as bar code scanners. More particularly this invention relates to a reflector arrangement in an illumination system in a bar code scanner.
2. Description of the Prior Art
Modern bar code reading devices are required to read relatively wide bar code symbols while remaining in a stationary position at the instant of reading. It is also desirable that the system optics provide a large depth of field in order to read curved surfaces, or surfaces otherwise having uncertain displacement from the reading device, and to generally provide increased ease of reading. At the same time there is a need to conserve power, a goal which is limited by the requirements of the optics, the sensitivity of the photodetector, and the minimal acceptable signal-to-noise ratio.
In one prior art example, Chadima Jr. et al, U.S. Pat. No. 4,766,300, a portable bar code reader employs an elliptical reflector that is elongated in a width dimension of the bar code reader so as to illuminate a region located in front of an optical window and containing a bar code symbol. In this device light rays diverge from a flash tube illumination source toward a surface to be scanned, and are reflected in a receive path that passes above and beyond the reflector in a generally rearward direction. This arrangement results in a relatively large angle between the illumination path and the receive path. Also, across the scan width there is significant off-axis falloff of illumination intensity, which limits the ability to read indicia that are situated at the ends of a wide sensing region.
In Yasuda, U.S. Pat. No. 4,891,739, an optical arrangement is shown for uniformly illuminating a planar surface using a curved reflecting mirror by a non-Lambertian arcuate or point light source. A light shielding member is required to prevent light from the source from directly reaching the object to be illuminated.